Signal addition to a wave division multiplex system

ABSTRACT

A telecommunications system is formed from a single mode optical fiber carrying Wave Division Multiplex (WDM) traffic. A transmission fiber coupler is arranged to couple signals from channels to be added to the single mode optical fiber. A coupler connects the channels carrying the signals to be added to the input of an optical amplifier, the output of the amplifier being connected in series to the transmission fiber coupler by a switchable multiband band-stop filter arranged to pass only signals having the wavelength of a signal or signals to be added to the WDM traffic. The switchable multiband band-stop filter may be a fiber Bragg filter, a selectable stack of filters or an assembly of bleachable filters. A switchable multiband filter is formed from a stack of layers of a semiconductor bleachable medium whereby the bleaching threshold is that of each single layer and the attenuation is the sum of the transmissions through all the layers comprising the stack.

Communications traffic is increasing year by year by around 100% in someareas due to internet, mobile telephony, interactive entertainment,video conferencing and communications, and information systems. Opticalfibres are being operated with many different wavelength channels inwavelength division multiplexed (WDM) systems. These fibres are beingused in communications networks in which traffic may be carried ondifferent carrier wavelengths through several switching points. Theseoptical networks may be constructed from optical WDM line systemsconnected by optical switches and from optical WDM rings interconnectedto allow traffic to be selectively switched between rings. A convenientway to construct such rings is from a basic building block at whichwavelength traffic channels may be added or dropped from the ring. Suchan equipment is called an Optical Add Drop Multiplexer for ringnetworks. Optical Add Drop Multiplexer functions are also used in WDMline systems to permit a fraction of the WDM channels to be dropped atintermediate points.

Adding and dropping of wavelength channels to a single mode fibre can beaccomplished by means of broadband splitter/combiners such as fibrefused couplers or silica waveguides formed in pairs and run with smalldimensional spacingless than a wavelength so that coupling between theoptical fields occurs. When such couplers are used, splitting/couplinglosses are very severe. For example, with two way coupling, loss is morethan 50% and for 32 way coupling losses are more than 97%.

Alternatively coupling can be achieved using diffractive and dispersiveelements to make wavelength division multiplexing combiners (WDMcombiners). Such WDM combiners may have n input ports and one outputport. To couple into the output port it is necessary to introduce eachwavelength channel into its correct port. Such devices have, inpractice, coupling losses between 1 dB (˜80%) to 7 dB (˜20%) dependingon quality and on band pass characteristics of the filtering of eachchannel.

A communications network becomes most economic when it becomes possibleto load up all parts of the network and when it is possible to providealternative ‘protection’ paths for traffic. This ideal is approachedwhen traffic can be easily switched from one wavelength channel toanother and when all switch interconnection options are available, iewhen the switches are ‘non-blocking’.

If wavelength charging is introduced it becomes necessary to haveswitches associated with the WDM combiners. Alternativelysplitter/combiners can be used but then optical amplifiers becomenecessary to overcome the large losses incurred during thesplitter/combiner functions. The amplifiers introduce ‘noise’ due toAmplified Spontaneous Emission (ASE) onto the traffic paths. Thismanifests as reduced optical signal to noise ratio (OSNR) in the opticalsignal carrying the communications traffic.

According to the present invention there is provided atelecommunications system comprising a single mode optical fibrecarrying Wave Division Multiplex (WDM) traffic and including atransmission fibre coupler arranged to couple signals from channels tobe added to the single mode optical fibre, further comprising couplingmeans to connect the channels carrying the signals to be added to theinput of an optical amplifier, the output of the amplifier beingconnected in series to the transmission fibre coupler by a switchablemultiband band-stop filter arranged to pass with low loss only signalshaving the wavelength of a signal or signals to be added to the WDMtraffic and to attenuate all signals at wavelengths not having thewavelength of the signal or signals to be added to the WDM traffic.

There is further provided a switchable multiband filter comprising astack of layers of a semiconductor bleachable medium whereby thebleaching threshold is that of each single layer and the attenuation isthe sum of the transmissions through all the layers comprising thestack.

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIGS. 1( a) to 1(f) show methods by which traffic can be added at anoptical add/drop-node;

FIG. 2 shows the use of a switchable multiband band-stop filter;

FIG. 3 shows a fibre Bragg switchable multiband band-stop filter;

FIG. 4 shows a switchable multiband band-stop filter formed from afilter stack;

FIG. 5 shows a switchable multiband band stop filter formed from adiffraction grating stack; and

FIG. 6 shows a switchable multiband band-stop filter formed usingbleachable media FIGS. 1( a) to 1(f) show methods by which traffic canbe added at an Optical Add Drop Node and comments regarding each methodare included:

FIG. 1( a) shows a method where channels are connected to a WDMmultiplexer and then to a transmission fibre coupler. This method can bescaled to large numbers of channels (WDM couplers for 80 channels ormore are available commercially today). The losses may be 3 dB for acoupler and 3 dB for a WDM multiplexer, a total of 6 dB and do notchange significantly with channel count (number of channels).

FIG. 1( b) shows where the channels are connected to an n-way waveguideor a fibre splitter/combiner functioning as a coupler. There is a lossof 1/n due to the multiway coupler and a loss of ½ due to thetransmission fibre coupler, a loss of 12 dB for an 8-way coupler and 3dB for the transmission fibre coupler, a total of 15 dB and losses riserapidly with channel court.

FIG. 1( c) provides a means to bypass the transmission coupler loss.This requires n off 2×2 switches between a pair of back-to-back WDMmultiplexers at each channel add/drop, where n is the channel count.

FIG. 1( d) shows how through traffic where required passes through aswitchable filter. This filter is equipped to selectively attenuate eachthrough wavelength channel and to heavily attenuate or effectivelyblock, selected channels from which traffic may have been dropped and/oronto which new traffic is to be added. Scaling to large channel countsresults in linearly scaling losses with this arrangement if a broadband(non wavelength selective) combiner is used, so high power opticaltransmitters are necessary.

FIG. 1( e) shows how ‘flexibility’ can be provided, that is anywavelength can be added to any input port, if a WDM multiplexer, anoptical switch and a transmission fibre coupler are used. The loss is½×½×½=⅛. Currently low loss switches are expensive. The whole switchmust be included to afford ‘flexibility’ even if only a few channels arerequired to be added. The switches may for example be 3-D typeMicro-Electrical Machine in Silicon (MEMS) which scale to large portcounts with low loss. The optical multiplexer may also have low loss forlarge channel counts. Hence this approach scales but total loss is stillsignificant and the cost and complexity of such an implementation islarge.

FIG. 1( f) shows how flexibility can be achieved with couplers as inFIG. 1( b) with an amplifier overcoming the splitter/coupler losses. Asthe channel count is increased so the gain has to be raised and thenoise added to the added channels and to the “through” trafficincreases.

To summarise:

FIG. 1( a) does not provide flexibility;

FIG. 1( b) introduces high loss and therefore requires higher powertunable laser sources;

FIG. 1( c) like FIG. 1( a) does not provide flexibility. It alsorequires switches with low cross talk;

FIG. 1( d) has the same problems as FIG. 1( b);

FIG. 1( e) provides flexibility but requires an n by n switch which isnot widely available and which has to be provided even when adding onlyone channel.

FIG. 1( f) has an amplifier which overcomes the loss but it introducesamplified spontaneous emission noise into the path of the ‘through’channels and so compromises the performance of the system.

The invention seeks to provide flexibility, gain to the source andsuppression of the ASE noise.

It is proposed that wavelength channels are added using a waveguide orfibre coupler, an amplifier and a switchable multiband band-stop filter,as shown in FIG. 2.

Traffic is coupled into the single mode fibre. It is amplified alongwith all the other channels. Noise is added because of the amplifiedspontaneous emission in the amplifier. The switchable filter is set upso as to pass only the wavelength channels to be added. The switchablefilter elements are set to have high loss in the wavelength bands of thechannels which are not being added. The filter than attenuates thebroadband amplified spontaneous emission from the amplifier. Thisreduces the noise added to the ‘through’ traffic channels.

Numerical modelling has shown that attenuation of the ASE becomesimportant for bit rates of 2½ and 10 Gbit/s and above when traffic isrequired to pass through several OADM nodes in a ring. The OSNR (OpticalSignal to noise Ratio) of traffic passing through several (say 8) OADMnodes may be improved by several dB by using the switchable multibandband-stop filter. The use of the amplifier in the add channel pathenables the number of add channels to be scaled to 32 and beyond. Theamplifier allows lower power transmitter modules to be used. Theswitchable filter is required to attenuate the ASE by ˜10 dB to 15 dB.Filter transmission loss for through channels of ˜3 dB would be viable.This approach allows low power tunable laser based transmitters to beused to add up to 32 channels and more. The channels can be added as thetraffic builds so equipment can be added and financed as required on a‘pay as you grow’ basis—or a “partial provisioning with growth asneeded” basis. For very large channel count—2 stages of combining,amplification and filtering may be used to keep the ASE within the limitto permit transmission through several nodes with acceptable signal tonoise OSNR ratio.

The switchable multichannel filter may be implemented in one of a numberof ways:

1) Fibre Bragg gratings, one for each channel, each grating tunable bytemperature or strain. The fibre gratings have a bandwidth around half awavelength channel spacing and can be tuned to pass or block thetraffic. These fibre filters are produced to be arranged in a seriesconfiguration as shown in FIG. 3. The temperature needs to be raised byaround 40 degrees Centigrade to tune by 50 GHz to allow traffic to passor be attenuated. Alternatively, strain can be applied by means of apiezoelectric actuator or by magnetostriction. A specification for thistype of filter is included in the tables below:

Switchable Blocker Specification

Specification Comment Optical Specifications No. of Channels 32 ChannelSpacing 100 GHz Frequency λ range 1535.82– 1560.61 nm Switching Time <1s Switching Range, 50 GHz ~0.4 nm Frequency Blocking Range >15 dBAdditional to insertion loss in pass state. Over channel width asspecified (27.5 GHz). Insertion Loss in <3 dB Note other channels may beany pass state combination of pass or block. Over channel width asspecified. Dispersion in pass 100 ps/nm Over channel width as specified.state maximum Flatness in band 0.5 dB Over channel width as specified.(pass state only) Insertion Loss <1 dB Over 32 channels. UniformityChannel Width >λ 0.11 nm Centred on ITU channel (>27.5 GHz total)Mechanical Specifications Dimensions 180 mm × 100 mm × 25 mm ElectricalSpecifications Power Consumption <10 W Environmental Temperature 0 to 70degrees C. Qualification Relevant Telecordia Reliability MTBF > 10⁵hours Vibration Relevant ETSI and NEBS

This filter requires the reflection to be electrically retuned. Topermit a channel to pass the reflector grating has to be tuned to sit inthe wavelength band between two channels. This is restrictive on thepacking of channels into a particular band. Also it needs programming,calibration and temperature control.

2) A filter pack can be placed in the path of the wavelength channels asshown in FIG. 4, actuators removing filters from the pack so as to allowpassing of the wavelength channels to be added and the ASE in the bandsof the other channels being attenuated. This is difficult to arrangemechanically and requires a precise fixed pass-band filter foe eachchannel.

3) Alternatively, diffraction gratings can be used and moved out of thebeam as required or switched off if an active grating medium such asLithium Niobate or Liquid Crystal or other electro-optic material isused as shown in FIG. 5.

4) Following a combiner and an amplifier as shown in FIG. 6, the signalis introduced into a wavelength dispersive system such as an ArrayedWave Guide optical multiplexer as described by M. Smit and Dragoni or adiffraction grating based optical multiplexer. For example there isdescribed the implementation in the diffraction grating demultiplexercase. The optical traffic is formed into a parallel beam incident on thediffraction grating in the optical arrangement. The diffracted beams arethen imaged as separate channels on to a reversible bleachable opticalmedium backed by a reflective element. Where the optical intensity ishigh, i.e. when an add channel is present, the medium bleaches andbecomes transparent and the mirror surface behind reflects thiswavelength back into the optical system which couples it back into thefibre where it is coupled by a circulator into the transmission fibre.When there is no add channel present the element blocks ASE noise atthat wavelength.

An example of a bleachable medium is Erbium doped P₂O₃ glass. Erbium canhave a high concentration in P₂O₃ glass. 1 mm thick plate could have afew dB of loss. The radiative lifetime of erbium atoms is ˜10⁻² secondsso once bleaching has occurred, it would not distort the digitallymodulated signal which may have pulse lengths ˜sub nanoseconds.Alternatively suitable dyes in polymer films are potential media forthis as long as bleach lifetimes are significantly longer than the bitperiod of the traffic being passed through the filter. Alsosemiconductor bleachable media comprising Cadmium Telluride, or CadmiumMercury Telluride, or Indium Gallium Arsenide Phosphide dad betweenIndium Phosphide layers for a double hetero-structure layer could beused. Here the absorber would be a semiconductor with bandgap less thanthe photon energy of the traffic and the intermediate (cladding) layerswill have wider band gaps. A particularly favourable bleachable materialfor this purpose is a multilayer stack of InP/InGaAs/InP/InGaAs in whichthe Indium Gallium Arsenide layers are made 0.02 μm thick and the InPlayers separating the InGaAs layers 0.01 to 0.03 μm thickness. Amaterials specification is given in the table below:

Material Quantum (including Composition Thickness Well Dopant type LayerNo. grades) spec. (see below) (nm) Repeats and conc. Tolerances: 22 InP1000 p = 1 e 18 Wavelength +/− 10 nm 21 GalnAs 20 NUD (nom. Thickness+/− 5% Undoped) Doping +/− 20% 4–20(even) InP 15 8 NUD Mismatch < +/−500ppm 3–19(odd) GalnAs 20 8 NUD  2 InP 10 n = 1 e 17  1 InP 2000 n = 1 e18 Substrate InP S.I. and n+ substrate n = 1 e 18

The attenuation of ASE is increased by having more GaInAs layers—10 ismodelled to give 15 dB attenuation for the reflection geometry describedabove. The GaInAs layer will bleach when power increases to ˜100 w/cm².If each channel is imaged to a spot of 8 μm diameter, then the bleachpower will be ˜50 μw. With 4μ diameter spot size the bleach power wouldbe ˜12 μw.

Instead of the diffractive grating an Arrayed Wave Guide (AWG) opticalmux could be used. A transmission configuration having a mux and demuxstage could also be used effectively. Then no circulator would berequired but lower net attenuation (from the single pass through thebleachable layer) would result and no reflector would be required.

Definitions

ASE:—Amplified Sportaneous Emission—the added noise from an opticalamplifier

AWG:—Arrayed Wave Guide—these are optical waveguides in a circuitdesigned for optical multiplexing and demultiplexing and useinterferenceto achieve dispersive wavelength separation

MUX:—Multiplexing device—a device to combine several signal channelsinto one

OSNR:—Optical Signal to Noise Ratio

Channel:—This has been used to mean a modulated optical carrier from asingle laser. The laser wavelength is selected to conform to aparticular tolerance within a standard grid—the ITU 100 GHz or 50 GHzStandard Grids for example

Wavelength:—this has been used to embrace a particular value and theband of wavelengths within one channel

Traffic:—refers in general to the data and analogue signals beingcarried by the transmission system

Bleachable filter:—A filter which passes optical beams having powersufficient to change the material absorption with low attenuation andsignificantly attenuates beams of lower power density. It is necessarythat the change in absorption is reversible and not brought about byactual damage such as physical hole burning.

REFERENCES

M. K. Smit: “New focusing and dispersive planar component based on anoptical phased array”, Electronics Letters, vol. 24, no. 7, pp.385–386,Mar. 1988.

A. R. Vellekoop and M. K. Smit: “Four-channel integrated-opticwavelength demuliplexer with weak polarisation dependence”, Journal ofLightwave Technology, vol. 9, no. 3, pp. 310–314, Mar. 1991.

C. Dragone: “An N×N optical multiplexer using a planar arrangement oftwo star couplers”, Photonics Technobgy Letter, vol. 3, no. 9, pp.812–815, September 1991.

1. A telecommunications system, comprising: a single mode optical fibercarrying wave division multiplex (WDM) traffic, a transmission fibercoupler for coupling signals from channels to be added to the singlemode optical fiber, coupling means for connecting the channels carryingthe signals to be added to an input of an optical amplifier, theamplifier having an output connected in series to the transmission fibercoupler by a switchable multiband band-stop filter operative for passinglow loss only signals having a wavelength of a signal or signals to beadded to the WDM traffic, and for attenuating all signals at wavelengthsnot having the wavelength of the signal or signals to be added to theWDM traffic, and the band-stop filter being operative for attenuatingsignals which are generated as a result of amplified spontaneousemission (ASE).
 2. The telecommunications system as claimed in claim 1,wherein the switchable multiband band-stop filter comprises a series offiber Bragg grating filters.
 3. The telecommunications system as claimedin claim 1, wherein the switchable multiband band-stop filter comprisesa stack of selective narrow band pass filters, and actuator means fordisplacing respective filters to pass selected channels.
 4. Thetelecommunications system as claimed in claim 1, wherein the switchablemultiband band-stop filter comprises a stack of diffraction gratingswhich are removed or deactivated or activated to pass selected channels.5. The telecommunications system as claimed in claim 1, wherein theswitchable multiband band-stop filter comprises an optical wavelengthchannel demultiplexer for spatially separating wavelength components ofthe signal carried by the single mode fiber, and a bleachable reflectorfor intercepting separated wavelength components of the signal, and forpassing low loss only signals having the wavelength of the signal orsignals to be added to the WDM traffic, and for attenuating all signalsat wavelengths not having the wavelength of the signal or signals to beadded to the WDM traffic.
 6. The telecommunications system as claimed inclaim 5, further comprising a multiplexer for recombining the trafficinto the single mode optical fiber.
 7. The telecommunications system asclaimed in claim 5, wherein the bleachable reflector comprises erbiumdoped glass.
 8. The telecommunications system as claimed in claim 5,wherein the bleachable reflector comprises a thermally activatedbleachable medium.
 9. The telecommunications system as claimed in claim8, wherein thermal activation is provided by the optical signals. 10.The telecommunications system as claimed in claim 9, wherein the thermalactivation is provided by photon energy of the optical signals.
 11. Thetelecommunications system as claimed in claim 5, wherein the bleachablereflector comprises a semiconductor bleachable medium.
 12. Thetelecommunications system as claimed in claim 11, wherein thesemiconductor bleachable medium comprises one of cadmium telluride,cadmium mercury telluride, indium gallium arsenide phosphide, and indiumgallium arsenide clad between indium phosphide layers for a doublehetero-structure layer.
 13. The telecommunications system as claimed inclaim 8, where the bleachable medium is a stack of gallium indiumarsenide layers between indium phosphide or wider gap indium galliumarsenide phosphide layers, so that a bleaching threshold is that of eachsingle layer, and an attenuation is a sum of transmissions through themultiple layers.
 14. The telecommunications system as claimed in claim6, wherein the multiplexer and the demultiplexer comprise an arrayedwave guide (AWG).
 15. The telecommunications system as claimed in claim5, further comprising an optical circulator.